The present invention relates to cooling of electronic components used for the receipt of electromagnetic signals.
More particularly, the invention provides an apparatus for cryogenic cooling of filters and amplifiers, such cooling improving the signal-to-noise ratio of radio frequency (RF) signals received by an antenna of the type typically used in relay stations servicing cellular telephones.
Powerful and sensitive antennas are required to receive the weak electromagnetic signals emitted at a distance, by cellular phones or by communication devices. Essential electronic components for such purpose are filters to cut out noise and amplifiers for increasing signal strength. A radiation shield is also helpful in cutting out unwanted incoming thermal radiation and in reducing heat gain of the electronic components and so to reduce the load on the cooler.
In practice satisfactory results are obtained by the use of superconducting filters matched with a low-noise amplifier array. High temperature superconducting filters operate well when cooled to a temperature of between 60-80K. The amplifier array and radiation shield also require cooling, although these components operate safisctorily in a 90-110K temperature range.
Prior art cooling devices use a single stage cryogenic cooler, which is arranged to operate at a temperature of about 60 K in order to meet the demands of filter cooling. The amplifiers and radiation shield are also thermally connected to the same single source of cooling, and although these latter items are cooled to a lower than necessary temperature, they function in a satisfactory manner.
The disadvantages of such equipment lies in the high power required to operate the cooler, and in its corresponding size, weight, first cost and running cost As the working temperature of any refrigerator is lowered, the work required to transfer a given amount of heat increases. This is evident from the following formula. If W equals the work required to extract the heat Q at a low temperature t and reject it at a higher temperature T, the Camot relationship holds that W=Q[(T-t)/t].
A further disadvantage in cooling the radiation shield and the amplifier array to a lower than necessary temperature lies in the fact that these components are responsible, for different reasons, for most of the cooling load. The amplifier array because it is an active heatgenerating device, and the radiation shield because of its substantial size. Furthermore, during start up the cooler is delayed in achieving the lower temperature required by the filters during the time it is extracting heat from the amplifier array and radiation shield.
In a previous U.S. Pat. No. 5,197 295 the present inventor disclosed a double stage Stirling cryogenic cooler with an infra red (IR) focal plane array detector directly mounted on top of the cooler's expander second stage, and with the radiation shield assembly mounted and cooled by the expander first stage to a temperature range of 200-100 K. This specification, relating to the higher frequency and smaller wavelength of IR equipment did not teach or suggest such cooling method to solve the problems involved in equipment such as superconducting filters, or for the lower frequencies and larger wavelengths used by radio waves.